Navigate to the folder where you want to add files, click upload files:

Select the file to upload and select 'create a new branch':

Then if that's the only file, add a change summary and click create pull request:

IF there are more files, go back to your fork, and click "branches", eg:

Select the branch you just created:

Be careful as you navigate now that the URL continues to have your branch name in it, if not then go back and select your branch. Now browse to where you want to upload files and click upload files. Repeat this process until you have them all uploaded.